1. Field of the Invention
The present invention relates to a vibration wave motor which is driven by a travelling vibration wave.
2. Description of the Prior Art
A vibration wave motor which is driven by a travelling wave motor and which has been recently put into practice is explained with reference to FIG. 1, in which numeral 1 denotes an electrostrictive element such as PZT (tinatate zirconum acid lead) and numeral 2 denotes a vibration member mode of an elastic material to which the electrostrictive element 1 is bonded. The vibration member 2 and the electrostrictive element 1 are held on a base 9 through a vibration absorber 4. Numeral 3 denotes a movable member which is rotatably supported by a stationary shaft 10 through a bushing 14. The stationary shaft 10 has its lower end mounted to the base 9 and its upper end threaded. A spring 16 is fitted to the stationary shaft 10 above the bushing 14 and a nut 18 is screwed over a washer 17. In this manner, the movable member 3 is urged to the vibration member 2.
FIG. 2 is a side elevational view showing a relation between the electrostrictive element 1 and the vibration member 2. The electrostrictive element 1 has a plurality of devices 1a.sub.1, 1a.sub.2, 1a.sub.3, . . . and 1b.sub.1, 1b.sub.2, 1b.sub.3, . . . bonded thereto and the group of electrostrictive devices 1a.sub.1, 1a.sub.2, 1a.sub.3, . . . and the group of electrostrictive devices 1b.sub.1, 1b.sub.2, 1b.sub.3, . . . are arranged to be shifted by one quarter of wavelength .lambda. of the vibration wave from each other. In the group of electrostrictive devices 1a.sub.1, 1a.sub.2, 1a.sub.3, . . . , the devices are arranged at a pitch of one half of the wavelength with adjacent devices being oppositely polarized. Symbols + and - indicate polarities. In the other group of electrostrictive devices 1b.sub.1, 1b.sub.2, 1b.sub.3, . . . , the devices are arranged at a pitch of one half of the wavelength with adjacent devices being oppositely polarized. Alternatively, one electrostrictive element which is as large as the array of those electrostrictive devices may be polarized at the above pitch. Electrodes for applying a voltage are formed on the opposite polarized surfaces of the electrostrictive element by evaporation or printing.
In the vibration wave motor thus constructed, a voltage is applied from an A.C. power supply 20 thickness wise (in the direction of polarization) of the electrostrictive element 1 is shown in FIGS. 3 and 4. In FIG. 3, when a positive (forward HA) voltage is applied from the drive power supply 20 to the electrostrictive device 1a.sub.2 in the direction of polarization, the electrostrictive device 1a.sub.2 expands in the direction of electric field or thickness wise and shrinks in the direction normal to the direction of electric field (arrow A). Since the voltage of the opposite polarity is applied to the adjacent electrostrictive element 1a.sub.1, it shrinks in the direction of electric field and expands in the direction normal to the direction of electric field (arrow B). In this manner, the respective electrostrictive devices expand and shrink. Since the vibration member 2 is bonded in union to the electrostrictive element 1, the expansion and shrinkage are propagated to the vibration member 2 so that it is bent as shown in FIGS. 4A and 4B. FIG. 4A shows a bending state when the forward voltage is applied to the electrostrictive device 1a.sub.2 and the reverse voltage is applied to the electrostrictive device 1a.sub.1. FIG. 4B shows a bending state when the reverse voltage is applied to the electrostrictive device 1a.sub.2 and the forward voltage is applied to the electrostrictive device 1a.sub.1.
An A.C. voltage V.sub.0 sin .omega.t is applied to one group of electrostrictive devices 1a.sub.1, 1a.sub.2, 1a.sub.3, . . . of the electrostrictive element and an A.C. voltage V.sub.0 cos .omega.t is applied to the other group of electrostrictive devices 1b.sub.1 1b.sub.2, 1b.sub.3, . . .. Accordingly, A.C. voltages having a phase difference of 180.degree. between adjacent devices in the polarization direction and a phase difference of 90.degree. between the groups are applied to the respective electrostrictive devices, and the electrostrictive devices expand and shrink. As this vibration is propagated to the vibration member 2, the vibration member 2 makes a bending vibration in accordance with the pitch of arrangement of the electrostrictive devices. When the vibration member 2 protrudes at the positions of the alternate electrostrictive devices, it recesses at the positions of other alternate electrostrictive devices. On the other hand, since one group of electrostrictive devices are shifted by one quarter of wavelength relative to the other group and the phases of the bending vibrations are shifted by 90.degree. from each other, the vibration waves are combined and travel. During the application of the A.C. voltage, the vibrations are sequentially generated and propagated through the vibration member 2 as the travelling bending vibration wave.
The waves under this condition are shown in FIGS. 5A, 5B, 5C and 5D. Let us assume that the travelling bending vibration wave travels in a direction of an arrow X.sub.1. A center plane of the vibration member in a quiescent state in shown by 0 and that in a vibration state is shown by a chain line. In the neutral plane 6, the bending stress is balanced. In sectional planes 7.sub.1 7.sub.2, 7.sub.3 ; and normal to the neutral plane 6, no stress is applied on a crossing line 5, of those planes and it merely vibrates vertically. The sectional plane 7.sub.1 makes a lateral pendulum motion around the crossing line 5.sub.1. In FIG. 5A, a point P.sub.1 on a crossing line of the sectional plane 7.sub.1 and the surface of the vibration member 2 facing the movable member 1 is a right dead center of the lateral vibration and vibrates only vertically. In this pendulum motion, when the crossing lines 5.sub.1, 5.sub.2 and 5.sub.3 are on the positive side of the wave (above the center plane 0), a leftward (opposite to the travel direction X.sub.1 of the wave) stress is applied, and when they are on the negative side of the wave (below the center plane 0), a rightward stress is applied. In FIG. 5A, the crossing line 5.sub.2 and the sectional plane 7.sub.2 show the former state and the stress shown by an arrow is applied to a point P.sub.2, and the crossing line 5.sub.3 and the sectional plane 7.sub.3 show the latter state and the stress shown by an arrow is applied to a point P.sub.3. As the wave travels and the crossing line 5.sub.1 comes to the positive side of the wave as shown in FIG. 5B, the point P.sub.1 moves leftward and upward. In FIG. 5C, the point P.sub.1 moves only leftward at the upper dead center of the vertical vibration. In FIG. 5D, the point P.sub.1 moves leftward and downward. As the wave further travels, the state of FIG. 5A is restored through the rightward and downward motion and the rightward and upward motion. Through the combination of the series of motions, the point P.sub.1 makes a rotating elliptic motion. As shown in FIG. 5C, the rotating elliptic motion is in a direction shown in FIG. 5C on a tangential line of the point P.sub.1 to the movable member 3 and the movable member 3 is frictionally driven in a direction X.sub.2 by the motion of the point P.sub.1. All points on the vibration member 2 sequentially frictionally drives the movable member 3 as the point P.sub.1 does.
This vibration wave motor does not have a performance which sufficiently meets requirements for a drive efficiency and durability. One of the causes therefor is that the electrostrictive element 1 is bonded to the vibration member 2. The vibration member 2 makes the bending vibration because only one side of the rigid vibration member 2 which is hardly expanded or shrinked is expanded or shrinked in the plane direction by the electrostrictive element 1. Accordingly, in the prior art vibration wave motor, it is necessary that the electrostrictive element and the vibration member are securely bonded in union. When such a bonding layer exists, a resonant frequency is not constant or .theta. is lowered. This is undesirable from the standpoint of the drive efficiency. Further, since a stress is always applied to the bonding layer during the drive, the bonding layer is apt to be peeled off. This is undesirable from the standpoint of durability.